To identify and isolate genes involved in the growth and differentiation of cells in the mammalian embryo and fetus, we carried out insertional mutagenesis studies in transgenic mice derived from retrovirally infected embryonic stem (ES) cells. The 412 strain segregates 23 proviruses, 3 of which cause recessive prenatal lethality. The molecular and phenotypic analysis of two of these, the 412-k and the 412-r mutations, is currently underway. We have isolated candidate genes for the 412-r mutation using the techniques of "exon-trapping" and RACE. These have been shown to be expressed during normal embryonic development. Currently we are addressing whether expression of any of these candidates is affected in homozygous 412-r mutant embryos. To accomplish this study, we developed methods to accurately genotype individual embryos using PCR. Similar approaches are being applied to the 412-k mutation. We have carefully studied the aberrant phenotype produced by the 412-k mutation. Mutants show specific cranial neural tube anomolies as well as general growth retardation. The 412-k mutation may represent an animal model for human anencephalies. A similar analysis is underway for the 412-r mutant.